Hydrothermal synthesis of polyimide-linked covalent organic frameworks towards ultrafast and stable cathodic sodium storage

Redox-active organic materials are capturing growing attention as cathode materials for sustainable alkaline metal ion batteries. However, the storage of Na⁺ in most organic materials-based cathodes is plagued by low capacity and unsatisfying rate performance due to their low active site densities and limited exposed active sites. Herein, two polyimide-linked covalent organic frameworks (COFs), namely HATN-PD-COF and HATN-TAB-COF, were fabricated from hydrothermal synthesis with redox-active triphenylene-2,3,6,7,10,11-hexacarboxylic acid and aromatic amines as starting materials. Powder X-ray diffraction and electron microscopy analysis indicate the high crystalline nature of these COFs with AA stacking configuration and orderly mesoporous tunnel. N₂ sorption measurement discloses the permanent porosity of these two COFs with a Brunauer-Emmett-Teller surface area of 1,065–1,200 m² g⁻¹ and a large pore size of 2.0–3.1 nm. Galvanostatic intermittent titration technique and density functional theory calculations reveal the facile Na⁺ ion diffusion along the mesoporous tunnel of these COFs with a small energy barrier of 0.13–0.40 eV. In particular, the as-prepared COFs based-cathodes show ultrafast and stable Na⁺ storage associated with their conjugated electronic structure, highly ordered mesoporous tunnel, robust structure, and redox-active C=N/C=O-rich framework as exemplified by the high reversible capacity of 210 mA h g⁻¹ at 200 mA g⁻¹, record-high rate performance (195 mA h g⁻¹ at a high current density of 10,000 mA g⁻¹) among organic electrodes and the capacity retention of nearly 91% at 10,000 mA g⁻¹ after 7,000 cycles for HATN-PD-COF.